1. Field of the Invention
This invention relates to an iron nitride magnetic powder for use in a high recording density magnetic recording medium and a method of producing the powder.
2. Background Art
In order to achieve the increasingly higher recording density required by today's magnetic recording media, efforts are being made to enable use of shorter recording wavelengths. For this, it is necessary to make the magnetic particle size much smaller than the length of the region for recording the short-wavelength signal. If it is not, a distinct magnetic transition cannot be produced, making practical recording impossible. The particle size of the magnetic powder is therefore required to be sufficiently smaller than the recording wavelength.
To realize higher recording density, the resolving power of the recording signal must be increased. Reduction of magnetic recording medium noise is therefore important. Noise is largely attributable to particle size. The finer the particles, the lower the noise becomes. This also makes it necessary for a magnetic powder used for high density recording to have sufficiently small particle size.
As the particles become finer, however, it becomes increasingly difficult for them to exist independently of each other. Even in the case of the metal magnetic powders widely used in data storage media, extreme particle refinement undesirably makes the powder susceptible to sintering during the reduction phase of the production process. When sintering occurs, the volume of the particles increases. They therefore become a source of noise and also adversely affect tape-making such as by degrading dispersibility and causing loss of surface smoothness. A magnetic powder suitable for a high-density recording medium requires good magnetic properties as a magnetic material but even more important are the powder properties it exhibits during tape-making, i.e., its particle size, particle size distribution, specific surface area, tap density, dispersibility and the like.
As taught by JP2000-277311A (Ref. No. 1) and W02003/079333A1 (Ref. No. 2), it is known that an iron nitride system magnetic powder whose main phase is Fe16N2 has excellent magnetic properties that make it suitable for a high-density recording medium. For example, Ref No. 1 describes an iron nitride magnetic material of large specific surface area that exhibits high coercive force (Hc) and high saturation magnetization (σs), and teaches that excellent magnetic properties can be achieved irrespective of shape owing to a synergistic effect between the magnetic anisotropy of an Fe16N2 phase and powder specific surface area enlargement. As improvements on the magnetic powder of Ref. No. 1, Ref. No. 2 proposes rare earth element-iron-boron system, rare earth element-iron system and rare earth element-iron nitride system magnetic powders composed of substantially spherical particles or ellipsoid particles and states that a tape medium produced using such a powder has excellent properties, that, in particular, the rare earth element-iron nitride system magnetic powder whose main phase is Fe16N2 exhibits a high coercive force of 2,500 (Oe) despite being composed of fine particles of about 20 nm diameter, high saturation magnetization owing to small BET specific surface area and also good storage stability, and that use of this rare earth element-iron nitride system magnetic powder enables a dramatic increase in the recording density of a coating-type magnetic recording medium. Ref. No. 2 further describes particles of a size level under 20 nm diameter, namely, of 17 nm diameter, having excellent magnetic properties.
This rare earth element-iron nitride system magnetic powder is produced by ammonia nitriding in which rare earth element-iron system magnetic powder obtained by reducing magnetite particles having rare earth elements and one or two of Al or Si adhered to the surface thereof is nitrided using NH3 gas. Owing to the large crystal magnetic anisotropy of the Fe16N2 phase produced by this nitriding, there can be obtained a magnetic powder suitable for a high recording density medium, i.e., a magnetic powder characterized by, for example, fine particle size, high Hc and high σs.
As pointed out in Ref. No. 1 and Ref. No. 2, this magnetic powder containing Fe16N2 phase that is small in average particle diameter and excellent in magnetic properties exhibits high potential as a magnetic material. However, these references are silent regarding its particle size distribution, dispersibility and other powder properties. This makes it difficult to judge whether the magnetic powder is suitable for the coating-type magnetic recording medium in which it is used. Even a magnetic powder excellent in magnetic properties is difficult to use in a coating-type magnetic recording medium if, for example, it is poor in surface smoothness.
Ref. No. 2 relates to generation of Fe16N2 phase having large crystal magnetic anisotropy in which fine particles that do not sinter are produced by adhering Si, Al, rare earth element(s) (defined as including Y) or the like to the particle surfaces as a sinter preventing agent. However, this sinter preventing method utilizing adhesion has a problem in that the particle size distribution of the obtained powder is poor when the adhesion conditions are inadequate because the difference in the degree of sinter preventing agent adhesion among the particles enables sinter prevention where adherence is good but allows sintering to occur where little sinter preventing agent is adhered. The problem is particularly severe in the case of fine particles because the particles tend to cohere and behave like aggregates, thereby aggravating unevenness of the adherence. Poor particle size distribution degrades the tape surface properties and, by extension, degrades the electromagnetic conversion characteristics of the tape.
Even if it should be possible to distribute the particles uniformly without coherence, the sinter prevention method relying on adhesion would still have the disadvantage that the amount of sinter preventing agent required for achieving total surface coating increases as the specific surface area of the particles increases with higher refinement. This leads to the problem of an increase in nonmagnetic components that reduces magnetization per unit quantity. In addition, when Si is used as a sinter preventing agent, although excellent sinter prevention effect can be achieved owing to the strong adsorption of Si, dispersion of the particles is undesirably inhibited by the strong self-bonding of the Si.